This invention relates to a process for improving the storage stability of alkali metal carbonate peroxyhydrates, hereinafter referred to as percarbonates, particularly sodium percarbonate, produced by known prior art processes.
Sodium percarbonate is used as a bleaching agent for detergent and cleaning agent compositions and as a sterilizing agent. Sodium percarbonate has advantageous dissolution properties and a high bleaching effect when used as a bleaching agent at low temperatures. Although sodium percarbonate is a compound which is chemically stable as such under standard conditions, it tends to recrystallize during prolonged storage, particularly under the influence of atmospheric humidity, and to form small needle-type crystallites which easily fuse with each other. This recrystallization during storage causes caking which increases if the stored sodium percarbonate is additionally subjected to pressure during storage. Caking can increase to such an extent during this process that an originally free-flowing sodium percarbonate produced by a known prior art process solidifies to form a block. Consequently, sodium percarbonate is not suitable for storage in silos, since recrystallization and caking occur particularly easily under the conditions necessary for silo storage (exchange of air, pressure exerted by the upper layers of product onto the lower layers of product) and the entire silo becomes blocked. In practice, sodium percarbonate is therefore collected after manufacture in large sacks with a capacity of approximately 1,000 kg and stored by the manufacturer in these until delivery. It is necessary to stack these large sacks in not more than two layers in order to prevent the sodium percarbonate from being subjected to excessive pressure. However, even with this type of intermediate storage, it is not possible to prevent sodium percarbonate from recrystallizing and caking. In order to assure a free-flowing product, it is thus necessary to empty the large sacks before delivery to the purchaser, to comminute the caked agglomerate of sodium percarbonate, and to repackage the comminuted product. It goes without saying that such a procedure comprising comminution and repacking of the sodium percarbonate crystals is extremely time-consuming and costly.
Prior art processes for chemically stabilizing sodium percarbonate, e.g. by introducing suitable stabilizers during the manufacture of the percarbonate from soda and hydrogen peroxide, or by subsequently coating the percarbonate with a suitable coating material such as boric acid, borates, magnesium silicates, etc., do not produce any significant improvement in this recrystallization and caking problem. Although the conventional stabilizers added during the production process and incorporated into the sodium percarbonate substantially stabilize the sodium percarbonate from the chemical point of view, they do not prevent recrystallization.
In addition, the rearrangement of water of crystallization associated with the recrystallization process also leads to the partial dissolution of the sodium percarbonate, making it possible for higher pH values to arise which causes additional decomposition of the sodium percarbonate. The conventional coating processes of the prior art also are incapable of suppressing recrystallization and caking. Moreover, it is difficult to coat percarbonate particles, since it is necessary to have a sufficiently smooth and homogeneous surface as free as possible from points and needle-shaped irregularities in order to successfully coat such particles.